Dip moldings, composition for dip molding and method for producing dip moldings

ABSTRACT

The present invention relates to a dip molding, a composition for dip molding, and a method for producing a dip molding. The dip molding is molded from a conjugated diene based rubber latex, crosslinked by a water-soluble multivalent metal salt, and contains substantially none of any of a sulfur-containing vulcanizing agent, a vulcanization accelerator for the vulcanizing agent, and zinc oxide. The dip molding has very few pinholes, has good feel, and has sufficient strength.

TECHNICAL FIELD

[0001] The present invention relates to a dip molding, a composition fordip molding, and a method for producing a dip molding and, moreparticularly, a dip molding that has very few pinholes, has good feel,and has sufficient strength, a composition for dip molding that issuitable as a starting material for the molding, and a method forproducing the molding efficiently.

BACKGROUND ART

[0002] Conventionally, rubber gloves are widely used in domestic work,various industries such as the food industry and electronic componentmanufacturing, and medical care (in particular, for surgery). The rubbergloves are required to be free of pinholes, have good feel so as toprovide ease of work, and not to break during work.

[0003] As the rubber gloves, those obtained by dip molding from anatural rubber latex are commonly used. However, there is a possibilitythat the natural rubber latex gloves might cause allergy in some usersdue to trace amounts of protein present in a rubber component and,therefore, gloves made of a synthetic rubber latex such as, for example,an acrylonitrile-butadiene copolymer latex, which are free of theabove-mentioned possibility, have been proposed.

[0004] U.S. Pat. No. 2,880,189 discloses a composition for dip molding,the composition comprising a water-insoluble multivalent metal oxide andan acrylonitrile-butadiene copolymer latex modified with a specificcarboxyl group neutralized with ammonia. Although a dip molding obtainedfrom such a composition for dip molding has very little possibility ofcausing allergy, since there are many pinholes it is necessary todispose of a considerable number of defective products by screening.Furthermore, the dip molding obtained from the composition for dipmolding, which contains no sulfur, tends to have a low tensile stress at300% elongation (good feel), but poor tensile strength (possibility ofbreaking during work).

[0005] WO 97/48765 discloses gloves that are dip-molded using acarboxyl-modified acrylonitrile-butadiene copolymer latex, ammoniumcasein, sulfur, and a vulcanization accelerator, and using no zincoxide. Although such a dip molding has a comparatively low tensilestress at 300% elongation and excellent tensile strength, there are manypinholes.

[0006] In the production of a dip molding, in order to vulcanize a dipmolding layer formed on the surface of a dip molding mold, the dipmolding layer is usually subjected to a vulcanization step in which itis thermally treated at a temperature of 100° C. to 130° C. for about 30minutes to about 1 hour. Such a thermal treatment requires a largeamount of thermal energy, and there is therefore a desire for a moreenergy-efficient production method.

DISCLOSURE OF INVENTION

[0007] It is an object of the present invention to provide a dip moldingthat has very few pinholes, has good feel, and has sufficient strength.It is another object of the present invention to provide a compositionfor dip molding that is suitable as a starting material for the abovemolding. It is yet another object of the present invention to provide amethod for producing the above molding with high productivity.

[0008] As a result of an intensive investigation by the presentinventors in order to achieve the above-mentioned objects, it has beenfound that controlling the pH of a conjugated diene based rubber latexobtained by copolymerization of a conjugated diene monomer and anethylenically unsaturated acid monomer can give a dip molding exhibitingsatisfactory performance without sulfur vulcanization, and the presentinvention has been accomplished based on such a finding.

[0009] That is, one aspect of the present invention provides thefollowing solution means.

[0010] A dip molding molded from a conjugated diene based rubber latexobtained by copolymerization of a conjugated diene monomer and anethylenically unsaturated acid monomer, wherein the dip molding iscrosslinked by a water-soluble multivalent metal salt, and molding iscarried out using substantially none of any of a sulfur-containingvulcanizing agent, a vulcanization accelerator for the vulcanizingagent, and zinc oxide.

[0011] Another aspect of the present invention provides the followingsolution means.

[0012] A composition for dip molding, the composition comprising aconjugated diene based rubber latex obtained by copolymerization of aconjugated diene monomer and an ethylenically unsaturated acid monomer,wherein the composition has a pH of at least 8.5, and containssubstantially none of any of a sulfur-containing vulcanizing agent, avulcanization accelerator for the vulcanizing agent, and zinc oxide.

[0013] Yet another aspect of the present invention provides thefollowing solution means.

[0014] A method for producing a dip molding comprising a step ofadhering to a dip molding mold a latex coagulating agent comprising awater-soluble multivalent metal salt, a step of forming a dip moldinglayer on the surface of the mold by immersing the mold in a compositionfor dip molding comprising a conjugated diene based rubber latexobtained by copolymerization of a conjugated diene monomer and anethylenically unsaturated acid monomer, the composition having a pH ofat least 8.5 and containing substantially none of any of asulfur-containing vulcanizing agent, a vulcanization accelerator for thevulcanizing agent, and zinc oxide, and a step of detaching the dipmolding layer after drying, in that order.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] The present invention is explained in detail below.

[0016] The ‘conjugated diene based rubber latex’ referred to in thepresent invention means a conjugated diene-ethylenically unsaturatedacid copolymer rubber latex obtained by polymerization of a monomermixture containing as essential components a conjugated diene monomerand an ethylenically unsaturated acid monomer. The monomer mixturepreferably contains, among the two types of monomers, the conjugateddiene monomer in a larger amount than that of the ethylenicallyunsaturated acid monomer. Furthermore, the monomer mixture may containan additional ethylenically unsaturated monomer that can copolymerizewith the above-mentioned two types of monomers.

[0017] The conjugated diene based rubber latex used in the presentinvention is preferably a copolymer latex obtained by polymerization ofa monomer mixture comprising 30 to 90 wt % of the conjugated dienemonomer, 0.1 to 20 wt % of the ethylenically unsaturated acid monomer,and 0 to 69.9 wt % of the additional ethylenically unsaturated monomerthat can copolymerize with the above monomers.

[0018] The conjugated diene monomer is not particularly limited, andexamples thereof include 1,3-butadiene, isoprene,2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-pentadiene, andchloroprene. These conjugated diene monomers may be used singly or in acombination of two or more types, and 1,3-butadiene or isoprene ispreferably used.

[0019] The amount of conjugated diene monomer used is preferably 30 to90 wt % of the monomer mixture, more preferably 40 to 85 wt %, andparticularly preferably 50 to 80 wt %. When it is less than 30 wt %, thedip molding obtained has a hard feel, and on the other hand when it ismore than 90 wt the gloves do not retain their shape and the tensilestrength tends to be low.

[0020] The ethylenically unsaturated acid monomer is not particularlylimited as long as it is an ethylenically unsaturated monomer having anacidic group such as a carboxyl group, a sulfonic acid group, or an acidanhydride group, and examples thereof include an ethylenicallyunsaturated monocarboxylic acid monomer such as acrylic acid ormethacrylic acid; an ethylenically unsaturated polycarboxylic acidmonomer such as itaconic acid, maleic acid, or fumaric acid; anethylenically unsaturated polycarboxylic acid anhydride such as maleicanhydride or citraconic anhydride; an ethylenically unsaturated sulfonicacid monomer such as styrenesulfonic acid; and an ethylenicallyunsaturated polycarboxylic acid partial ester monomer such as monobutylfumarate, monobutyl maleate, or mono-2-hydroxypropyl maleate. Theseethylenically unsaturated acid monomers can also be used in the form ofan alkali metal salt or an ammonium salt. These ethylenicallyunsaturated acid monomers may be used singly or in a combination of twoor more types. Among these ethylenically unsaturated acid monomers, itis preferable to use an ethylenically unsaturated carboxylic acid, it ismore preferable to use an ethylenically unsaturated monocarboxylic acid,and it is particularly preferable to use methacrylic acid.

[0021] The amount of ethylenically unsaturated acid monomer used ispreferably 0.1 to 20 wt % of the monomer mixture, more preferably 1 to15 wt %, and particularly preferably 2 to 6 wt %. When it is less than0.1 wt %, the tensile strength of the dip molding is low, and on theother hand when it is more than 20 wt %, the dip molding has a hardfeel.

[0022] With regard to the additional ethylenically unsaturated monomerthat can copolymerize with the conjugated diene monomer and theethylenically unsaturated acid monomer, examples thereof include anethylenically unsaturated nitrile monomer such as acrylonitrile,methacrylonitrile, fumaronitrile, α-chloroacrylonitrile, orα-cyanoethylacrylonitrile; a vinyl aromatic monomer such as styrene, analkylstyrene, or vinyinaphthalene; a fluoroalkyl vinyl ether such asfluoroethyl vinyl ether; an ethylenically unsaturated amide monomer suchas (meth)acrylamide, N-methylol(meth)acrylamide,N,N-dimethylol(meth)acrylamide, N-methoxymethyl (meth) acrylamide, orN-propoxymethyl(meth)acrylamide; an ethylenically unsaturated carboxylicacid ester monomer such as methyl (meth)acrylate, ethyl (meth)acrylate,butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, trifluoroethyl(meth)acrylate, tetrafluoropropyl (meth)acrylate, dibutyl maleate,dibutyl fumarate, diethyl maleate, methoxymethyl (meth)acrylate,ethoxyethyl (meth)acrylate, methoxyethoxyethyl (meth)acrylate,cyanomethyl (meth)acrylate, 2-cyanoethyl (meth)acrylate, 1-cyanopropyl(meth)acrylate, 2-ethyl-6-cyanohexyl (meth)acrylate, 3-cyanopropyl(meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate, glycidyl (meth)acrylate, or dimethylaminoethyl(meth)acrylate; and a crosslinking monomer such as divinylbenzene,polyethylene glycol di(meth)acrylate, polypropylene glycoldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, orpentaerythritol (meth)acrylate. These ethylenically unsaturated monomersmay be used singly or in a combination of two ore more types.

[0023] Among these monomers, it is preferable to use an ethylenicallyunsaturated nitrile monomer, and in particular acrylonitrile.

[0024] The amount of ethylenically unsaturated monomer used ispreferably 0 to 69.9 wt % of the monomer mixture, more preferably 0 to59 wt %, and particularly preferably 14 to 48 wt %. When it is more than69.9 wt %, the dip molding obtained has a hard feel.

[0025] The monomer mixture used for producing the conjugated diene basedrubber latex particularly preferably comprises 1,3-butadiene,acrylonitrile, and methacrylic acid, the composition thereof beingpreferably 40 to 79 wt %, 20 to 45 wt %, and 1 to 15 wt %, respectively,and more preferably 54 to 73 wt %, 25 to 40 wt %, and 2 to 6 wt %,respectively.

[0026] The conjugated diene based rubber latex used in the presentinvention is usually produced by an emulsion-polymerization method. Thepolymerization temperature during emulsion-polymerization is notparticularly limited, but it is particularly preferably 45° C. or lesssince the latex can be produced stably and a dip molding having highmechanical strength and a soft feel can be obtained. The polymerizationtemperature during emulsion-polymerization is more preferably in therange of 10° C. to 45° C.

[0027] In the production of the conjugated diene based rubber latex, themethod for adding the monomer mixture is not particularly limited, andany method can be employed such as a method in which the monomer mixtureis charged all at once into a polymerization reactor, a method in whichthe monomer mixture is continuously supplied to the polymerizationreactor, or a method in which part of the monomer mixture is chargedinto the polymerization reactor and the rest of the monomer iscontinuously supplied to the polymerization reactor.

[0028] A polymerization initiator used in the production of theconjugated diene based rubber latex is not particularly limited, butspecific examples thereof include an inorganic peroxy compound such assodium persulfate, potassium persulfate, ammonium persulfate, potassiumperphosphate, or hydrogen peroxide; an organic peroxy compound such asdiisopropylbenzene hydroperoxide, cumene hydroperoxide, t-butylhydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide,2,5-dimethylhexane-2,5-dihydroperoxide, di-t-butyl peroxide, di-α-cumylperoxide, acetyl peroxide, isobutyryl peroxide, or benzoyl peroxide; andan azo compound such as azobisisobutyronitrile,azobis-2,4-dimethylvaleronitrile, or methyl azobisisobutyrate. Thesepolymerization initiators may be used singly or in a combination of twoor more types. An inorganic or organic peroxy compound initiator ispreferably used since a latex can be produced stably and a dip moldinghaving a high mechanical strength and a soft feel can be obtained. Theamount of polymerization initiator used depends to some extent on thetype thereof, but it is preferably 0.01 to 1.0 parts by weight based on100 parts by weight of the monomer mixture.

[0029] The inorganic or organic peroxy compound initiator can be used incombination with a reducing agent as a redox polymerization initiator.This reducing agent is not particularly limited, and examples thereofinclude a compound containing a metal ion in a reduced state such asferrous sulfate or cuprous naphthenate; a sulfonate compound such assodium methanesulfonate; and an amine compound such as dimethylanilineor sodium ethylenediaminetetraacetate. These reducing agents may be usedsingly or in a combination of two or more types. The amount of reducingagent used depends to some extent on the type thereof, but is preferably0.03 to 10 parts by weight based on 1 part by weight of the peroxycompound.

[0030] Among these initiators, it is preferable to use the redoxpolymerization initiator in which the peroxy compound initiator and thereducing agent are combined.

[0031] An emulsifier used in the production of the conjugated dienebased rubber latex is not particularly limited, but examples thereofinclude a nonionic emulsifier such as a polyoxyethylene alkyl ether, apolyoxyethylene alkylphenol ether, a polyoxyethylene alkyl ester, or apolyoxyethylene sorbitan alkyl ester; an anionic emulsifier such as asalt of a fatty acid, for example, myristic acid, palmitic acid, oleicacid, or linolenic acid, a salt of an alkylbenzenesulfonic acid, forexample, sodium dodecylbenzenesulfonate, a higher alcohol sulfate ester,or an alkylsulfosuccinate salt; a cationic emulsifier such as analkyltrimethylammonium chloride, a dialkylammonium chloride, orbenzylammonium chloride; and a copolymerizable emulsifier such as asulfo ester of an α,β-unsaturated carboxylic acid, a sulfate ester of anα,β-unsaturated carboxylic acid, or a sulfoalkyl aryl ether. An anionicemulsifier or a nonionic emulsifier is particularly preferably used.These emulsifiers may be used singly or in a combination of two or moretypes. Since the amount of emulsifier used affects the chemicalstability of the latex for dip molding, it is usually used in a range of0.1 to 9 wt % based on the monomer mixture.

[0032] In the emulsion polymerization, a polymerization adjuvant(polymerization auxiliary) such as a molecular weight adjusting agent, aparticle size adjusting agent, an antioxidant, a chelating agent, or anoxygen scavenger can be used as necessary.

[0033] The particle size of the conjugated diene based rubber latex ispreferably 60 to 300 nm, and more preferably 80 to 150 nm, as anumber-average particle size measured by observation using atransmission electron microscope. This particle size can be adjusted toa desired value by, for example, regulating the amounts of emulsifierand polymerization initiator used.

[0034] The composition for dip molding of the present inventioncomprises such a conjugated diene based rubber latex, and can beobtained by adjusting the pH thereof to at least 8.5, preferably 9.5 to13, and more preferably 10.5 to 12, and making it contain substantiallynone of any of a sulfur-containing vulcanizing agent, a vulcanizationaccelerator for the vulcanizing agent, and zinc oxide. When the pH ofthe composition for dip molding is less than 8.5, the strength of thedip molding is degraded, and it is very difficult to adjust the pH sothat it is more than 13.

[0035] The sulfur-containing vulcanizing agent referred to in thepresent invention is a material that can crosslink high molecular weightchains of a plastic conjugated diene based rubber into a net-likestructure, and sulfur is representative thereof. The sulfur-containingvulcanizing agents can be roughly divided into inorganicsulfur-containing vulcanizing agents and organic sulfur-containingvulcanizing agents; specific examples of the former include sulfur(powdered sulfur, flowers of sulfur, acidless sulfur, precipitatedsulfur, colloidal sulfur, polymeric sulfur, insoluble sulfur) and sulfurmonochloride, and specific examples of the latter include those that canrelease active sulfur by thermal dissociation, such as morpholinedisulfide and an alkylphenol disulfide. Specific examples of otherorganic sulfur-containing vulcanizing agents are listed in ‘Gomu KogyoBinran (Rubber Industry Handbook) 4th Edition, III ChemicalAdditives, 1. Vulcanizing Agents’, ed. by the Society of RubberIndustry, Japan, (published by the Society of Rubber Industry, Japan,January, 1994).

[0036] The vulcanization accelerator referred to in the presentinvention is a material that accelerates a vulcanization reaction in thepresence of a sulfur-containing vulcanizing agent, thereby exhibitingeffects in shortening the vulcanization time, lowering the vulcanizationtemperature, or decreasing the sulfur-containing vulcanizing agent.Specific examples of the vulcanization accelerator include athiazole-based vulcanization accelerator such as zinc2-mercaptobenzothiazole, a thiourea-based vulcanization accelerator suchas diphenylthiourea, a guanidine-based vulcanization accelerator such asdiphenylguanidine, and aldehyde/ammonia and aldehyde/amine-basedvulcanization accelerators such as hexamethylenetetramine. Specificexamples of other vulcanization accelerators are listed in ‘Gomu KogyoBinran (Rubber Industry Handbook) 4th Edition, III Chemical Additives,3. Vulcanization Accelerators’, ed. by the Society of Rubber Industry,Japan, (published by the Society of Rubber Industry, Japan, January,1994).

[0037] The presence of a sulfur-containing vulcanizing agent increasesthe number of pinholes in a dip molding. ‘Containing substantially none’of the sulfur-containing vulcanizing agent means particularly preferablycontaining none at all, but it can be in a range that does not impairthe objects of the present invention, for example, preferably at most0.4 parts by weight based on 100 parts by weight of the solids portionof the conjugated diene based rubber latex, more preferably at most 0.2parts by weight, yet more preferably less than 0.1 parts by weight, andparticularly preferably less than 0.05 parts by weight.

[0038] The presence of a vulcanization accelerator for the vulcanizingagent increases the number of pinholes in a dip molding. ‘Containingsubstantially none’ of the vulcanization accelerator means particularlypreferably containing none at all, but it can be in a range that doesnot impair the objects of the present invention, for example, preferablyat most 0.4 parts by weight based on 100 parts by weight of the solidsportion of the conjugated diene based rubber latex, more preferably atmost 0.2 parts by weight, and yet more preferably less than 0.1 parts byweight.

[0039] The presence of zinc oxide increases the number of pinholes in adip molding. ‘Containing substantially none’ of the zinc oxide meansparticularly preferably containing none at all, but it can be in a rangethat does not impair the objects of the present invention, for example,preferably at most 0.7 parts by weight based on 100 parts by weight ofthe solids portion of the conjugated diene based rubber latex, morepreferably at most 0.3 parts by weight, and yet more preferably at most0.1 parts by weight.

[0040] ‘Molding being carried out using substantially none of any of asulfur-containing vulcanizing agent, a vulcanization accelerator for thevulcanizing agent, and zinc oxide’ or ‘containing substantially none ofany of a sulfur-containing vulcanizing agent, a vulcanizationaccelerator for the vulcanizing agent, and zinc oxide’ meansparticularly preferably using or containing none of any of asulfur-containing vulcanizing agent, a vulcanization accelerator for thevulcanizing agent, and zinc oxide, but they can be contained in a rangethat does not impair the objects of the present invention, for example,based on 100 parts by weight of the solids portion of the conjugateddiene based rubber latex, the sulfur-containing vulcanizing agent beingat most 0.4 parts by weight, the vulcanization accelerator for thevulcanizing agent being at most 0.4 parts by weight, and the zinc oxidebeing at most 0.7 parts by weight, and more preferably thesulfur-containing vulcanizing agent being at most 0.2 parts by weight,the vulcanization accelerator for the vulcanizing agent being at most0.2 parts by weight, and the zinc oxide being at most 0.3 parts byweight.

[0041] In order to adjust the pH of the composition for dip molding ofthe present invention, a basic substance is added. Examples of the basicsubstance include a hydroxide of an alkali metal such as lithiumhydroxide, sodium hydroxide or potassium hydroxide; a carbonate of analkali metal such as sodium carbonate or potassium carbonate; a hydrogencarbonate of an alkali metal such as sodium hydrogen carbonate; ammonia;and an organic amine compound such as trimethylamine or triethanolamine.The alkali metal hydroxides and/or ammonia are particularly preferable,and the alkali metal hydroxides are more preferable. These basicsubstances are usually added in the form of a solution, and preferablyan aqueous solution, having a concentration of 1 to 40 wt %, andpreferably 2 to 15 wt %, in order to prevent formation of aggregatesduring addition.

[0042] With regard to a method for adjusting the pH of the compositionfor dip molding, examples thereof include a method in which the pH ofthe conjugated diene based rubber latex is adjusted in advance and amethod in which the pH is adjusted after mixing the conjugated dienebased rubber latex with another additive if desired, but the method isnot particularly limited as long as the pH of the composition for dipmolding can finally be in the desired range.

[0043] The total solids content of the composition for dip molding isusually 5 to 50 wt %, preferably 10 to 45 wt %, and more preferably 20to 40 wt %. When this content is too low, it is difficult to obtain adip molding having a desired thickness, and on the other hand when it istoo high, it is difficult to handle the composition for dip molding dueto its high viscosity, or the thickness of the dip molding tends to beuneven.

[0044] The composition for dip molding of the present invention maycontain a standard additive such as an antioxidant, a dispersant, aviscosity increasing agent, a pigment, a filler, or a softening agent ina range that does not impair the objects of the present invention. It isalso possible to use, in combination, another latex such as a naturalrubber latex or an isoprene rubber latex as long as the objects of thepresent invention are not impaired.

[0045] It is not necessary for the composition for dip molding of thepresent invention to be subjected to an aging step that is usuallycarried out for a conventional composition for dip molding. Since a dipmolding having sufficient strength can be obtained without carrying outaging, the production of a dip molding is simple.

[0046] The water-soluble multivalent metal salt used in the presentinvention is a salt of a metal of groups 2, 12, and 13 of the periodictable and has a solubility of at least 5 parts by weight in 100 parts byweight of water at 25° C., and preferably at least 20 parts by weight.The water-soluble multivalent metal salt has the function of degradingthe stability of latex particles dispersed in an aqueous phase tothereby coagulate the latex particles.

[0047] Specific examples of the water-soluble multivalent metal saltinclude a halide such as barium chloride, calcium chloride, magnesiumchloride, zinc chloride, or aluminum chloride; a nitrate such as bariumnitrate, calcium nitrate, or zinc nitrate; an acetate such as bariumacetate, calcium acetate, or zinc acetate; and a sulfate such as calciumsulfate, magnesium sulfate, or aluminum sulfate. In particular, calciumchloride and calcium nitrate are preferable. These water-solublemultivalent metal salts are preferably used in the form of an aqueoussolution. The concentration of the aqueous solution depends on the typeof water-soluble multivalent metal salt, but is usually 5 to 70 wt %,and preferably 20 to 50 wt %. The temperature of the aqueous solutionwhen used is usually 0° C. to 90° C., and preferably 30° C. to 70° C.

[0048] The dip molding of the present invention can be obtained by aproduction method comprising a step of forming a dip molding layer onthe surface of a dip molding mold using a composition for dip moldingand a latex coagulating agent comprising a water-soluble multivalentmetal salt, and a step of detaching the dip molding layer after drying,in that order.

[0049] The step of forming a dip molding layer can be roughly dividedinto an anode coagulant dipping process and a Teague coagulant dippingprocess. It is preferable to form a dip molding layer by the formermethod because the dip molding thus obtained has a less uneventhickness.

[0050] The anode coagulant dipping process comprises a step of immersinga dip molding mold in a latex coagulation liquid comprising awater-soluble multivalent metal salt so as to adhere the latexcoagulation liquid to the surface of the mold, and a step of immersingthe mold in the composition for dip molding of the present invention soas to form a dip molding layer on the surface of the mold, in thatorder. The Teague coagulant dipping process comprises a step ofimmersing a dip molding mold in the composition for dip molding of thepresent invention so as to form a coating of the composition for dipmolding on the surface of the mold, and a step of immersing the mold ina latex coagulation liquid comprising a water-soluble multivalent metalsalt so as to form a dip molding layer on the surface of the mold, inthat order. It is also possible to form a dip molding layer having alarge film thickness by repeating one of the two methods or repeatingthe two methods alternately.

[0051] The mold temperature of the dip molding mold and the temperatureof the composition for dip molding when carrying out dip molding areusually room temperature to 90° C., and preferably 40° C. to 80° C. Theperiod of time during which the dip molding mold is immersed in thelatex coagulation liquid and the period of time during which the dipmolding mold to which the latex coagulation liquid has been adhered isimmersed in the composition for dip molding in the anode coagulantdipping process can be adjusted appropriately so that a dip moldinglayer having a desired thickness can be obtained.

[0052] The dip molding layer thus obtained is preferably washed withwater before drying. The washing with water here means immersing the dipmolding layer in water, and preferably hot water at 40° C. to 70° C.,for about 2 to about 60 minutes so as to remove water-soluble impurities(for example, surpluses of the emulsifier, water-soluble multivalentmetal salt, etc.) This operation may be carried out after drying the dipmolding layer, but is preferably carried out before drying sincewater-soluble impurities can be removed more efficiently. Carrying outthis operation can outstandingly improve the tensile strength of the dipmolding.

[0053] The dip molding layer thus obtained has a high water content andneeds to be dried. The drying is carried out so that the water contentof the dip molding layer becomes preferably 5 wt % or less, and morepreferably 2 wt % or less. With regard to the drying method, a methodinvolving external heating by means of infrared radiation or hot air, orinternal heating by means of high-frequency waves can be employed. Thedrying by means of hot air is particularly preferable. The dryingtemperature is usually 60° C. to 95° C., and preferably 70° C. to 85°C., and the drying time is usually about 10 to about 120 minutes.

[0054] Since the drying can impart sufficient strength to the dipmolding layer, detaching this layer from the dip molding mold can give adip molding. As a method for detachment, a method in which it isdetached from the dip molding mold by hand or a method in which it isdetached by means of water pressure or compressed air pressure isemployed.

[0055] Prior to detachment, it may be subjected to a thermal treatmentat a temperature of 100° C. to 150° C. for 10 to 120 minutes, but thecomposition for dip molding of the present invention can give a dipmolding having sufficient strength by only the above-mentioned drying.Furthermore, subsequent to detachment, it may be subjected to a furtherthermal treatment at a temperature of 60° C. to 120° C. for 10 to 120minutes if desired.

[0056] In the dip molding of the present invention, the ratio of thenumber of moles of the multivalent metal originating from thewater-soluble multivalent metal salt used relative to the number ofmoles of the acid group of the bonded ethylenically unsaturated acid inthe conjugated diene based rubber latex is preferably 0.2 to 1.5, morepreferably 0.3 to 1.3, and particularly preferably 0.35 to 0.8. Whenthis ratio is in this range, the dip molding has excellent tensilestrength.

[0057] This ratio is determined by dividing a value obtained byincinerating the dip molding at a high temperature and converting theincinerated portion into a metal chloride by hydrochloric acid so as toquantitatively determine the number of moles of the multivalent metaloriginating from the included water-soluble multivalent metal salt, bythe number of moles of the acid group of the bonded ethylenicallyunsaturated acid in the conjugated diene based rubber latex forming thedip molding. When quantitatively determining the number of moles of themultivalent metal, if there is contamination by a component that doesnot originate from the water-soluble multivalent metal salt but can bequantitatively determined as the same multivalent metal, the amountthereof is corrected before the calculation.

[0058] This ratio can be adjusted to a desired value by appropriatelyadjusting the amount of bonded ethylenically unsaturated acid in theconjugated diene based rubber latex, the pH of the composition for dipmolding, the concentration of the water-soluble multivalent metal salt,and/or the dipping conditions, etc.

[0059] The dip molding of the present invention can easily achieve atensile stress at 300% elongation of less than 4.5 MPa, preferably lessthan 3.5 MPa, and more preferably less than 3.0 MPa, excellent feel, anda tensile strength of at least 20 MPa, preferably at least 25 MPa, andmore preferably at least 30 MPa.

[0060] The tensile stress and the tensile strength referred to here meanthose measured by a method described in examples.

[0061] The dip molding of the present invention can be produced so as tohave a thickness of about 0.1 to about 3 mm, and can be usedparticularly suitably as a thin product having a thickness of about 0.1to about 0.3 mm. Specifically, it is suitable as a medical product suchas a baby's bottle nipple, a dropper, a tube, or a hot-water bottle; atoy or sports equipment such as a balloon, a doll, or a ball; anindustrial product such as a pressure molded bag or a gas storage bag;surgical, domestic, agricultural, fishery, and industrial gloves; and afinger stall. It is particularly suitable as thin surgical gloves.

[0062] The present invention is explained in further detail below byreference to examples. ‘Parts’ and ‘%’ in the examples are on a weightbasis unless otherwise specified.

[0063] (Properties Evaluation Methods)

[0064] (pH)

[0065] Measured at 25° C. using a pH meter (M12: manufactured by Horiba,Ltd.). The latex measured had a solids content of 45%, and thecomposition for dip molding measured had a solids content of 30%.

[0066] (Tensile Stress at 300% Elongation and Tensile Strength)

[0067] In accordance with ASTM-D412, a dip-molded rubber glove wasstamped using a Die-c dumbbell to make a test piece. The test piece wassubjected to measurements of tensile stress at 300% elongation andtensile strength at break at a tensile speed of 500 mm/min using aTensilon universal tester (RTC-1225A: manufactured by KK Orientech).

[0068] The smaller the tensile stress at 300% elongation, the better thefeel of the dip molding.

[0069] (Rubber Glove Transparency)

[0070] The transparency of a dip-molded rubber glove was evaluatedvisually.

[0071] (Number of Pinholes)

[0072] A rubber glove was filled with water, and after 30 minutes hadpassed, the number of pinholes was counted by defining as one pinhole apoint where the water inside was leaking on the outer surface of theglove. 100 rubber gloves were prepared per sample, and the total numberof pinholes formed in all the rubber gloves was measured.

[0073] (Amount of Bonded Methacrylic Acid in Conjugated Diene BasedRubber Latex)

[0074] Part of a copolymer latex after completion of the polymerizationreaction was sampled, residual unreacted monomer was removed, and asolid rubber was then obtained by coagulation and drying according toJIS K 6392.

[0075] About 0.2 g of the solid rubber sample was precisely weighed anddissolved in 100 ml of pyridine, and the solution thus obtained wassubjected to neutralization titration of carboxyl groups in thecopolymer with a 0.2 N alcoholic solution of potassium hydroxide underan atmosphere of nitrogen using thymolphthalein as an indicator. Theamount (wt % basis) of bonded methacrylic acid in the solid rubber wasdetermined from the amount of potassium hydroxide required to neutralizethe carboxyl groups in the solid rubber.

[0076] (Multivalent Metal/Bonded Methacrylic Acid Molar Ratio in DipMolding)

[0077] 0.5 g of a dip molding was precisely weighed, placed in a metalcrucible, and treated in an electric furnace at 600° C. for 2 hours soas to incinerate it. The ash portion remaining in the metal crucible wasthen dissolved using 10 g of a 0.1 N aqueous solution of hydrochloricacid, and the concentration of the multivalent metal chlorideoriginating from the water-soluble multivalent metal dissolved in thesolution was measured by capillary electrophoresis with an aqueoussolution of calcium chloride as a calibration curve.

[0078] The measurement conditions of the capillary electrophoresis wereas follows. Measurement equipment: CAPI-3200 Capillary ElectrophoresisSystem (manufactured by Otsuka Electronics Co., Ltd.) Buffer: 2 mMimidazole 5 mM 2-hydroxyisobutyric acid 100 μl acetic acid/100 mldistilled water Wavelength: 215 nm Applied voltage: 25 kV Measurementmethod: Indirect method

[0079] The number of moles (R) of the multivalent metal was obtainedusing equation (1) below. $\begin{matrix}{R = \frac{10 \times C}{100 \times M_{CaCl2}}} & (1)\end{matrix}$

[0080] (In the equation, C: concentration of the multivalent metalchloride dissolved in 10 g of the 0.1 N hydrochloric acid (wt %: calciumchloride basis), M_(Cacl2): molecular weight of calcium chloride=111.)

[0081] In the case where the conjugated diene based rubber latex alonewas used, the number of moles (M) of bonded methacrylic acid containedin the dip molding used here was obtained using equation (2) below.$\begin{matrix}{M = \frac{{0.5 \times {amount}\quad \left( {{wt}\quad \%} \right){of}\quad {bonded}\quad {methacrylic}\quad {acid}\quad {in}\quad {the}\quad {solid}\quad {rubber}}\quad}{100 \times 86}} & (2)\end{matrix}$

[0082] In the case where a component other than the conjugated dienebased rubber latex was contained in the composition for dip molding, theamount of rubber component in the dip molding was corrected according tothe amount of additional component.

[0083] The multivalent metal/bonded methacrylic acid molar ratio in thedip molding was obtained by dividing the value for R by the value for M.

PRODUCTION EXAMPLES 1 TO 6

[0084] A pressure-resistant polymerization reactor was flushed withnitrogen and charged with a total of 100 parts of the monomers shown inTable 1, 0.5 parts of t-dodecylmercaptan as the molecular weightadjusting agent, 132 parts of soft water, 1.5 parts of sodiumdodecylbenzenesulfonate as the emulsifier, 0.3 parts of potassiumpersulfate as the polymerization initiator, and 0.05 parts of sodiumethylenediaminetetraacetate as the reducing agent, a reaction wascarried out at a polymerization temperature of 37° C. for 30 hours, andthe polymerization was then terminated by adding 0.1 parts of sodiumdimethyldithiocarbamate as a reaction terminator. The polymerizationconversion was at least 97% in all cases.

[0085] After removing unreacted monomer from the latex thus obtained,the pH of the copolymer latex was adjusted with 28% aqueous ammonia togive conjugated diene based rubber latexes A to F having a solidscontent of 45% and a pH of 8.0. TABLE 1 Production Example 1 2 3 4 5 6Monomer composition (parts) Acrylonitrile 37 37 37 20 28 371,3-Butadiene 58 59.5 61.5 72 68 63 Methacrylic acid 5 3.5 1.5 8 4 —Conjugated diene based rubber latex A B C D E F Amount of bondedmethacrylic acid 5.2 3.4 1.4 8.3 4.2 0 (wt %)

EXAMPLE 1

[0086] A composition for dip molding was prepared by adjusting the pH ofthe conjugated diene based rubber latex A to 10.0 using a 5% aqueoussolution of potassium hydroxide and the solids content to 30%.

[0087] A dip molding glove mold heated at 60° C. was dipped for 10seconds in an aqueous solution of latex coagulating agent obtained bymixing 20 parts of calcium nitrate as the water-soluble multivalentmetal salt, 0.05 parts of a nonionic emulsifier (Emulgen-810:manufactured by Kao Corporation), and 80 parts of water, then pulled outand dried at 60° C. for 10 minutes so as to adhere the water-solublemultivalent metal salt to the mold surface.

[0088] This glove mold coated with the coagulating agent was thenimmersed in the above-mentioned composition for dip molding for 15seconds and then pulled out so as to form a dip molding layer on thesurface of the glove mold. This was dried at 20° C. for 5 minutes andfurther at 80° C. for 20 minutes to give a solid coating on the surfaceof the glove mold. Finally, this solid coating was peeled from the glovemold to give a glove-shaped dip molding having a thickness of 0.1 to 0.2mm (the same applies below). The results of evaluation of this dipmolding are given in Table 2.

EXAMPLE 2

[0089] The procedure of Example 1 was repeated except that the pH of thecomposition for dip molding was adjusted to 11.0 using a 5% aqueoussolution of potassium hydroxide. The results are given in Table 2.

EXAMPLE 3

[0090] The procedure of Example 2 was repeated except that, after dryingat 80° C. for 20 minutes, a dip molding was obtained by a furtherthermal treatment at 120° C. for 20 minutes. The results are given inTable 2.

EXAMPLE 4

[0091] The procedure of Example 2 was repeated except that, beforedrying at 80° C. for 20 minutes, the glove mold having the dip moldinglayer was immersed in distilled water at 50° C. for 5 minutes so as toremove water-soluble impurities. The results are given in Table 2.

EXAMPLES 5 AND 6

[0092] The procedure of Example 4 was repeated except that each of theconjugated diene based rubber latexes B and C shown in Table 1 was usedinstead of the conjugated diene based rubber latex A. The results aregiven in Table 2.

EXAMPLES 7 AND 8

[0093] The procedure of Example 4 was repeated except that each of theconjugated diene based rubber latexes D and E shown in Table 1 was usedinstead of the conjugated diene based rubber latex A. The results aregiven in Table 2.

EXAMPLE 9

[0094] As a white pigment, a titanium oxide liquid dispersion wasprepared by mixing 0.1 parts of titanium oxide, 0.05 parts of a 40%aqueous solution of the sodium salt of aβ-naphthalenesulfonate-formaldehyde condensate, and 0.1 parts of waterin a ball mill. 0.25 parts of this liquid dispersion and 250 parts(corresponding to 100 parts solids content) of the conjugated dienebased rubber latex E were mixed, the pH thereof was then adjusted to11.0 using a 5% aqueous solution of potassium hydroxide, and the solidscontent thereof was adjusted to 30% to give a composition for dipmolding.

[0095] The procedure of Example 8 was repeated except that thiscomposition for dip molding was used. The results are given in Table 2.

COMPARATIVE EXAMPLE 1

[0096] A vulcanizing agent dispersion was obtained by mixing 1 part ofzinc 2-mercaptobenzothiazole, 1.5 parts of sulfur, 1 part of zinc oxide,0.5 parts of a 40% aqueous solution of the sodium salt of aβ-naphthalenesulfonate-formaldehyde condensate, and 3.4 parts of waterin a ball mill. 7.4 parts of this vulcanizing agent dispersion and 250parts (corresponding to 100 parts solids content) of the conjugateddiene based rubber latex A were mixed, the pH thereof was then adjustedto 10.0 using a 5% aqueous solution of potassium hydroxide, the solidscontent thereof was adjusted to 30%, and the mixture thus obtained wasaged for 1 day to give a composition for dip molding. The procedure ofExample 3 was repeated except that this composition for dip molding wasused. The results are given in Table 2.

COMPARATIVE EXAMPLE 2

[0097] A vulcanizing agent dispersion was obtained by mixing 1 part ofzinc 2-mercaptobenzothiazole, 1.5 parts of sulfur, 0.5 parts of a 40%aqueous solution of the sodium salt of aβ-naphthalenesulfonate-formaldehyde condensate, and 2.5 parts of waterin a ball mill. 5.4 parts of this vulcanizing agent dispersion and 250parts (corresponding to 100 parts solids content) of the conjugateddiene based rubber latex A were mixed, the pH thereof was then adjustedto 10.0 using a 5% aqueous solution of potassium hydroxide, the solidscontent thereof was adjusted to 30%, and the mixture thus obtained wasaged for 1 day to give a composition for dip molding. The procedure ofExample 3 was repeated except that this composition for dip molding wasused. The results are given in Table 2.

COMPARATIVE EXAMPLE 3

[0098] A zinc oxide dispersion was obtained by mixing 1 part of zincoxide, 0.5 parts of a 40% aqueous solution of the sodium salt of aβ-naphthalenesulfonate-formaldehyde condensate, and 0.5 parts of waterin a ball mill. 2.0 parts of this dispersion and 250 parts(corresponding to 100 parts solids content) of the conjugated dienebased rubber latex A were mixed, the pH thereof was then adjusted to10.0 using a 5% aqueous solution of potassium hydroxide, the solidscontent thereof was adjusted to 30%, and the mixture thus obtained wasaged for 1 day to give a composition for dip molding. The procedure ofExample 3 was repeated except that this composition for dip molding wasused. The results are given in Table 2.

COMPARATIVE EXAMPLE 4

[0099] The procedure of Example 1 was repeated except that a compositionfor dip molding was used in which the pH of the conjugated diene basedrubber latex A was not changed from 8.0 and the solids content wasadjusted to 30%. The results are given in Table 2. When peeling off theglove-shaped dip molding from the glove mold, the dip molding stretchedor broke, and a dip molding having a satisfactory glove shape could notbe obtained.

COMPARATIVE EXAMPLE 5

[0100] The procedure of Example 2 was repeated except that theconjugated diene based rubber latex F was used instead of the conjugateddiene based rubber latex A. The results are given in Table 2. Whenpeeling off the glove-shaped dip molding from the glove mold, the dipmolding stretched or broke, and a dip molding having a satisfactoryglove shape could not be obtained. TABLE 2 Example Comparative Example 12 3 4 5 6 7 8 9 1 2 3 4 5 Composition for dip molding Conjugated dienebased rubber latex A A A A B C D E E A A A A F Sulfur (parts) — — — — —— — — — 1.5 1.5 — — — Vulcanization accelerator (parts) — — — — — — — —— 1.0 1.0 — — — Zinc oxide (parts) — — — — — — — — — 1.0 0 1.0 — —Titanium oxide (parts) — — — — — — — — 0.1 — — — — — pH of composition10.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0 11.0 10.0 10.0 10.0 8.0 11.0Water-washing prior to drying No No No Yes Yes Yes Yes Yes Yes No No NoNo No Thermal treatment conditions Drying (80° C., 20 min) Yes Yes YesYes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Heating (120° C., 20 min) —— Yes — — — — — — Yes Yes Yes — — Multivalent metal/bonded methacrylic0.40 0.46 0.47 0.46 0.71 1.28 0.39 0.59 0.58 0.53 0.44 0.50 0.11 — acid(molar ratio) (Properties of dip molding) Stress at 300% elongation(MPa) 2.5 2.8 2.9 3.4 2.8 2.3 4.4 2.3 2.4 6.2 3.5 5.2 0.9 1.3 Tensilestrength (MPa) 24.5 27.3 27.5 45.6 39.8 28.4 43.5 35.2 35.8 28.4 24.323.7 4.5 3.5 Transparency of rubber glove* Trsp Trsp Trsp Trsp Trsp TrspTrsp Trsp Opq Opq Cldy Opq Trsp Trsp Number of pinholes (holes) 3 2 2 11 0 1 0 7 38 20 28 — —

[0101] The following was found from the results shown in Table 2.

[0102] A dip molding obtained from a composition for dip moldingcomprising sulfur, a vulcanization accelerator, and zinc oxide hadcomparatively good feel and good tensile strength, but had a very largenumber of pinholes (Comparative Example 1).

[0103] A dip molding containing no zinc oxide and obtained by sulfurvulcanization had good feel, but had low tensile strength and a largenumber of pinholes (Comparative Example 2).

[0104] A dip molding obtained from a composition for dip moldingcontaining neither sulfur nor a vulcanization accelerator hadcomparatively good feel but had low tensile strength and quite a largenumber of pinholes (Comparative Example 3).

[0105] A dip molding obtained from a composition for dip molding whichdid not contain sulfur, a vulcanization accelerator or zinc oxide andwhose pH was less than the range specified in the present invention hadexcellent feel but had very low tensile strength and was not suitable asa dip molding (Comparative Example 4).

[0106] A dip molding obtained from a conjugated diene based rubber latexin which an ethylenically unsaturated acid monomer was not copolymerizedhad excellent feel but had very low tensile strength and was notsuitable as a dip molding (Comparative Example 5).

[0107] In comparison with these comparative examples, the dip moldingsobtained from the compositions for dip molding as specified in thepresent invention had excellent feel, excellent tensile strength, a verysmall number of pinholes. The dip moldings of Examples 1 to 8 weretransparent, but it is expected that a colored molding could easily beproduced, if desired.

[0108] When Examples 2 and 3 are compared, it is found that a dipmolding having a sufficient tensile strength can be obtained without athermal treatment, which is necessary for vulcanization.

[0109] Water-washing prior to drying gave dip moldings having excellentfeel and outstandingly improved tensile strength (Examples 4 to 9).

INDUSTRIAL APPLICABILITY

[0110] The dip molding of the present invention has very few pinholes,good feel, and adequate strength. A transparent dip molding can also beobtained easily. The composition for dip molding of the presentinvention is suitable as a starting material of the production of thedip molding, and the production method of the present invention canproduce the dip molding with high productivity.

1. A dip molding molded from a conjugated diene based rubber latexobtained by copolymerization of a conjugated diene monomer and anethylenically unsaturated acid monomer, wherein the dip molding iscrosslinked by a water-soluble multivalent metal salt, and molding iscarried out using substantially none of any of a sulfur-containingvulcanizing agent, a vulcanization accelerator for the vulcanizingagent, and zinc oxide.
 2. The dip molding according to claim 1 whereinthe conjugated diene based rubber latex is obtained by copolymerizationof 30 to 90 wt % of the conjugated diene monomer, 0.1 to 20 wt % of theethylenically unsaturated acid monomer, and 0 to 69.9 wt % of anadditional ethylenically unsaturated monomer that can copolymerize withthese monomers.
 3. The dip molding according to claim 1 wherein, basedon 100 parts by weight of the solids portion of the conjugated dienebased rubber latex, the sulfur-containing vulcanizing agent is at most0.4 parts by weight, the vulcanization accelerator for the vulcanizingagent is at most 0.4 parts by weight, and the zinc oxide is at most 0.7parts by weight.
 4. The dip molding according to claim 1 wherein, basedon 100 parts by weight of the solids portion of the conjugated dienebased rubber latex, the sulfur-containing vulcanizing agent is at most0.2 parts by weight, the vulcanization accelerator for the vulcanizingagent is at most 0.2 parts by weight, and the zinc oxide is at most 0.3parts by weight.
 5. The dip molding according to claim 1 wherein themolding is carried out using none of any of the sulfur-containingvulcanizing agent, the vulcanization accelerator for the vulcanizingagent, and the zinc oxide.
 6. The dip molding according to claim 1wherein it has a tensile stress at 300% elongation of less than 4.5 MPaand a tensile strength of 20 MPa or more.
 7. The dip molding accordingto claim 1 wherein the ratio of the number of moles of the multivalentmetal originating from the water-soluble multivalent metal salt usedrelative to the number of moles of the acid group of the bondedethylenically unsaturated acid in the conjugated diene based rubberlatex is 0.2 to 1.5.
 8. A composition for dip molding, the compositioncomprising a conjugated diene based rubber latex obtained bycopolymerization of a conjugated diene monomer and an ethylenicallyunsaturated acid monomer, wherein the composition has a pH of at least8.5, and contains substantially none of any of a sulfur-containingvulcanizing agent, a vulcanization accelerator for the vulcanizingagent, and zinc oxide.
 9. The composition for dip molding according toclaim 8 wherein the pH is 9.5 to
 13. 10. A method for producing a dipmolding comprising a step of adhering to a dip molding mold a latexcoagulating agent comprising a water-soluble multivalent metal salt, astep of forming a dip molding layer on the surface of the mold byimmersing the mold in a composition for dip molding comprising aconjugated diene based rubber latex obtained by copolymerization of aconjugated diene monomer and an ethylenically unsaturated acid monomer,the composition having a pH of at least 8.5 and containing substantiallynone of any of a sulfur-containing vulcanizing agent, a vulcanizationaccelerator for the vulcanizing agent, and zinc oxide, and a step ofdetaching the dip molding layer after drying, in that order.
 11. Themethod for producing a dip molding according to claim 10 furthercomprising a step of water-washing the dip molding layer prior to dryingthe dip molding layer.